Big screen microdisplay-based rear projection imaging systems, such as televisions and monitors, are becoming increasingly popular with consumers. This is due to their combination of lightweight, high picture quality, price, and reliability over comparable large display technologies such as cathode ray tube (CRT) based rear projection imaging systems, plasma displays, and direct view liquid crystal displays (LCD). Microdisplay based rear projection imaging systems work by shining a light on single or multiple microdisplays (also known as imagers or light-valves) where an image is displayed. Microdisplays create this image by electronically turning “on” or “off” specific pixels within an array of pixels on the microdisplay's display surface. The properties of the light that shines through or on the pixels is altered based on the “on” or “off” state of the pixel. Microdisplays are typically either transmissive, where light shines through the display, or reflective, where light is reflected off the microdisplay. High temperature polysilicon (HTPS) is commonly used in transmissive microdisplays, and the most common reflective microdisplays are digital micromirror devices (DMD) and liquid crystal on silicon (LCOS). The light that shines through or off the microdisplays is directed by projection optics on to the back surface of a transmissive display screen, consequently showing the image from the microdisplay. In the process the microdisplay image is magnified, usually around one hundred times (100×) for a fifty-inch (diagonally measured) screen. An audience watches the image on the front surface of the display screen.
One drawback of microdisplay based rear projection imaging systems, at least compared to plasma and direct view LCD imaging systems, is the depth of the cabinet or enclosure. Plasma and direct view LCD imaging systems can potentially provide an imaging system with a fifty inch or larger screen in an enclosure that is a mere five inches “thin”, or deep. A microdisplay based rear projection imaging system with a comparable screen size is at least twice as deep. Cabinet depth is a very important consideration to consumers due to the large volumes of space big screen imaging systems consume.
Reducing the depth of a microdisplay based rear projection imaging system requires either the use of expensive, precision projection optics or the sacrifice of displayed picture quality. The use of precision optics lessens or negates the price advantage microdisplay based rear projection imaging systems have over plasma displays and LCD imaging systems. High precision optics are necessary in the thin rear projection imaging systems due to the close proximity of the projector to the viewing screen and the angle of the projector. The projection unit for a microdisplay imaging system usually sits below the horizontal centerline of the screen due to space concerns, with the projection unit projecting up on to the back surface of the screen. The thinner a microdisplay rear projection imaging system is, the more acute the angle is between light rays exiting the projection optics and the screen.
Complex optical distortions result due to this acute projection angle and the wide-angle, short focal length lenses and optics necessary to achieve high magnification in a very short distance. These spatial and lens requirements result in geometric distortions such as keystones, pincushion effects, and anamorphic effects. “Keystoning” is a term used to describe the way a square or rectangular image is warped into a trapezoidal shape, with parallel top and bottom sides but angled vertical sides, when an image is displayed from off-axis on to an angled surface. Anamorphic effects also result from off axis projection, and cause unequal magnification of above- and below-axis parts of the image. Pincushion effects result from the choice of projection optics. A “pincushion effect” warps an image so that the centers of parallel lines curve towards the center of the screen. All these effects, unless corrected for, combine together to result in a warped, distorted image on the display screen.
Correcting the image distortions associated with thin, microdisplay based rear projection imaging systems requires both high precision projection optics and precision manufacturing. The tight tolerances associated with the precision manufacturing requirements further increase manufacturing costs. In addition to the tight tolerances, every part of the projection system must be properly installed and aligned to very high tolerances and must remain in alignment for the lifetime of the imaging system to avoid image distortion.
Some of the cost of manufacturing thin, microdisplay based rear projection imaging systems can be mitigated by electronically pre-warping the image displayed by the microdisplay, and thus allowing the use of less expensive, lower precision optics. The microdisplay projects a pre-warped image through the projecting optics, and the projecting optics distort the image again, back to normal dimensions. The end result is a projected image that looks undistorted on the display screen. The electronics driving the microdisplay pre-warp an image by taking a signal from an input device (such as a television tuner) and applying a predetermined warp map to the image. The combination of this image signal and warp map transformation determines which pixels the microdisplay turns on and off. Computing the predetermined warp map is a complicated and involved process requiring computed three-dimensional ray traces.
Both the lens based solution and the pre-warping based solution to correcting geometric distortions on microdisplay rear projection imaging systems suffer potential problems once the imaging systems are manufactured. Both solutions require that the projection optics and microdisplays stay in very precise alignment with each other and the display screen. If a rear projection imaging system is dropped or jostled during shipping, or if mounting fixture material properties relax as the imaging system ages, the optics and microdisplays may come out of alignment and cause the aforementioned geometric distortions. Less severe alignment problems such as changes in humidity or barometric pressure can cause the enclosure cabinet or mounts to warp and cause the misalignment of the different units as well.
Chromatic aberrations, or shifts in the color of an image, can likewise result from both rough handling of the imaging system and environmental changes. As stresses on projection optics change due to mounting fixture warping, the physical shape of the lens may change, causing issues with color resolution. Stress birefringence resulting from mounting fixture warping impairs the way optics handle polarized light. This is a considerable problem with imagers that require the use of polarized light such as microdisplays like LCOS and HTPS. Chromatic shift problems may also occur over time as an illumination bulb ages.
Some of the post-manufacturing distortion problems can be corrected by service personnel who can re-align the optics, the microdisplay, or microdisplays within the imaging system. This re-alignment can only solve minor problems and must be performed by a trained technician. There is no easy way for a consumer to quickly calibrate or realign the projection optics in an imaging system. There is also no current method for correcting problems stemming from optics or projection systems that are manufactured slightly out of tolerance.
Accordingly, it is desirable to provide a rear projection imaging system with image warping distortion correction system and an associated method. The warping distortion correction system allows the distortion-free rear projection imaging system to be manufactured in a very thin enclosure without having to resort to expensive, precision made projection optics.